The present invention generally relates to aircraft electric taxi systems (ETS). More particularly, the invention relates to control of ground speed of an aircraft as the aircraft is taxied by the ETS.
A typical aircraft may taxi to and from runways with thrust force developed by its engines that are optimized for high speed at altitude operation not ground idle. A significant amount of fuel may be burned by the engines during a typical aircraft taxi profile before and after each flight. In many cases, the main engines may provide more motive force than is required to complete a successful taxi profile and brakes have to be applied to slow the aircraft down. The actual speed is determined by engine thrust, aircraft weight, rolling friction and atmospheric conditions. In that regard, engine-thrust taxiing may be considered inefficient and may contribute to high fuel costs and ground level emissions.
Nevertheless, many existing aircraft and many experienced pilots are familiar with using throttle controls to regulate ground speed of an aircraft during taxiing. It would be desirable to provide the benefits of ETS in existing aircraft and to employ a speed controls systems for an ETS that could be operated in a manner similar to existing thrust based taxiing controls. Indeed, it would be desirable to provide even more options for speed control than those that presently exist in thrust based systems.
The motors of an ETS have significantly less power that the jet engines they replace and are only intended for low speed operation. Therefore an aircraft taxiing with an ETS will accelerate more slowly than an aircraft that is taxiing under jet power. Thus a pilot using an ETS may employ maximum power to accelerate an aircraft during taxiing. But when a desired speed is achieved the pilot may not know an appropriate control-lever-positioning to maintain the desired speed. Slow acceleration makes it difficult to adjust if one gets it wrong.
As can be seen, there is a need for a ground speed control system that can be retrofitted into an aircraft along with an ETS. More particularly, there is a need for such a speed control systems that employs pilot interfaces that are similar to those which pilots are presently familiar. Moreover there is a need for controls which display both commanded and actual speed and which have the capability of holding a commanded speed.